The invention relates to a method for eliminating interference in an OFDM (Orthogonal Frequency Division Multiplexing) radio receiver which receives coded signals transmitted on subcarriers, reliability information generated for the signal being used in soft decoding of the coded signals.
In the MCM methods (Multi-Carrier Modulation), information to be transferred is divided into parallel bit streams, each of which having a number of bits that is only a fraction of the amount of information to be transferred. Parallel bit streams are used for modulating several different carriers.
One form of the MCM methods is the OFDM method (Orthogonal Frequency Division Multiplexing) described in U.S. Pat. No. 3,488,445. In OFDM, the parallel bit streams are modulated by using densely disposed subcarriers whose radio spectra partly overlap because steep band-pass filters separating subcarrier spectra are not used in the method. In the time domain, forms that are mutually orthogonal are chosen for the subcarriers, i.e. forms that can be separated from one another although the radio spectra of the subcarriers may overlap. The waveforms can be generated at a transmitter and they can be decomposed at a receiver by using mathematical fast Fourier transform (FFT) or the inverse form thereof.
In OFDM, wide bandwidths, for example 1.5 MHz, are typically used, and data transmission rates are also high, for example 1.7 Mbit/s. The number of subcarriers is large, typically more than a hundred, for example 180 subcarriers, per channel.
OFDM is used as a modulation method for new digital broadcasting DAB (Digital Audio Broadcasting). Research is being conducted to use the method in DVB (Digital TV Broadcasting). In fixed data transmission networks, the method is used for implementing high-capacity modems, in association with modems known as ADSL modems (Asymmetric Digital Subscriber Loop). Transmission rates as high as 2-6 Mbit/s are then achieved by using a conventional twisted-pair cable of a telephone subscription.
The OFDM method is also well suitable for use in mobile telephone systems. By using the method, services requiring a wide transmission band, such as packet data transmission or video image transmission, can be efficiently implemented.
Communication on a radio path may include narrow-band interference interfering with some subcarriers. Interference may originate from an adjacent channel, for example. When the OFDM method is used, interleaving and coding of information to be transmitted is therefore needed so as to obtain an acceptable bit error ratio level for the information to be transferred.
One of the most efficient coding methods is convolutional coding. In that case, soft decoding is used at the receiver. Soft means here that the value 0 or 1 is not directly set for a bit, but reliability information obtained from the received signal is utilized for estimating, for example, with a certain probability, whether the bit decision made was correct.
At a simple differential demodulation receiver not employing amplitude equalization, the reliability information is derived from the amplitudes of the demodulated bits, the amplitudes representing carrier, interference and noise.
However, the described manner of obtaining reliability information is not satisfactory, since high interference may entirely mask the desired signal, and the amplitude measured at the receiver may then be very high, whereby, correspondingly, a very high value is obtained as the reliability value even though the desired signal is entirely buried in the interfering signal. Decoding convolutional coding of said received signal then leads to errors.
An object of the invention is thus to provide a method and equipment implementing the method so as to solve the above mentioned problems. This is achieved by the method of the type presented in the introduction, which is characterized in that the signal-to-interference ratio of the subcarrier is used as the reliability information, the signal-to-interference ratio being defined by dividing the amplitude of the received signal by the interference amplitude which is defined by calculating the deviation of the amplitudes of a set of selected signals from the average amplitude level of the set of signals.
The invention also relates to an OFDM (Orthogonal Frequency Division Multiplexing) radio receiver comprising an antenna; radio frequency parts for receiving coded signals transmitted on subcarriers via the antenna; a demultiplexer for dividing the received signal into orthogonal subcarriers; a demodulator for demodulating each subcarrier signal into symbols; a reliability part for generating reliability information for the received signals; means for de-interleaving the symbols between different subcarriers; a decoding part for soft decoding the coding of the symbols by using the reliability information. The OFDM radio receiver of the invention is characterized in that the reliability part is arranged to generate the reliability information as the signal-to-interference ratio; to define the signal-to-interference ratio by dividing the amplitude of the received signal by the interference amplitude; to define the interference amplitude by calculating the deviation of the amplitudes of a set of selected signals from the average amplitude level of the set of signals.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea that when a modulation method not affecting the signal amplitude is used for generating symbols to be transmitted, it can then be assumed that the signal amplitude is constant. The amplitude remains almost constant between symbols sequential in the time domain, since the symbols are short. Propagation delay is small at least in a microcell, and consequently a channel response does not change rapidly in the frequency domain either, whereby the amplitudes are also assumed to remain almost constant between symbols in parallel subcarriers in the frequency domain. Amplitude variations thus indicate the effect of interference on the signal.
The method and system of the invention provide many advantages. Reliability of soft decoding is considerably improved. When interference occurs on a channel, the bit error ratio of the information transferred on the channel can be considerably improved by using the method of the invention. Information, such as packet data, requiring a demanding bit error ratio can then be transferred.